How do I get dark mode on a IPhone 5s

No it came with ios13

• • iOS 14.4.1 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max
• • iOS 12.5.1 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)

That is a feature of a much later iOS version not available to your phone

but to verify try this

1. Step 1: Open the setting app on your device.
2. Step 2: Scroll down to the display and brightness option.
3. Step 3: There you will see the option of light mode and dark mode.
4. Step 4: Click on the dark mode option

I cannot remember, is that available on ios12?